The present invention relates to antennas for wireless communication equipment, and particularly to improvements in compact plate antennas which are suitable for use as antennas for mobile or portable communication equipment.
A typical configuration for antennas for communication equipment or transceivers mounted aboard vehicles, or for mobile or portable communication equipment such as cordless telephones, has been the classical .lambda./4 monopole antenna as typified by the whip antenna. This is the most widespread type and has been used in most cases up to date. Here, .lambda. is the wavelength of the frequency f.
Generally speaking, when an antenna is raised to a higher elevation, it becomes proportionally less susceptible to the influences of the topography and surface objects and gains a higher sensitivity in the reception of incoming radio waves. However, as long the aforesaid monopole antennas were used in mobile or portable communication equipment such as those dealt with here, there were restrictions on their height. Since they could not be raised up very high, it was not always possible to achieve a desirable sensitivity.
It is also undesirable to position an antenna too low, and there is the limitation that the aforesaid .lambda./4 must be followed at the minimum. Even though there has been a tendency in recent communication equipment to miniaturize the circuit parts remarkably by adopting various types of integrated circuits, no progress has been made in miniaturization of antenna parts, and miniaturization has proved to be entirely unsuitable for the antennas of portable communication equipment which are carried around indoors by a person while speaking, such as the remote units of cordless telephones.
Monopole antennas also have problems in their basic principles of operation. Since the antennas are of the type sensitive to electric fields, they are easily susceptible to the influences of persons or other dielectric substances in the vicinity, and the antenna performance has sometimes deteriorated under the conditions of actual use.
Concerning this point, generally in mobile wireless communications, even if the waves are transmitted from the base station as vertically polarized waves, their plane of polarization becomes inclined as the waves are reflected and scattered by the topography, structures, etc. located in the path of propagation, so that horizontal polarization is sometimes stronger than vertical polarization in the waves when they arrive at a mobile station. This tendency is especially pronounced in cities where there are many tall buildings, steel towers, and the like.
The same may be said about wireless local intercommunication systems. Here also, the waves are reflected and scattered by the equipment installed, and by machines, implements, ceilings, columns, beams and the like, so that very often the waves arriving at a mobile station have a different plane of polarization from the waves which were transmitted.
For this reason, when monopole antennas are used in an attempt to deal with this polarization of the propagated radio waves, one must rely on the so-called polarization diversity effect, for example by positioning two monopole antennas, one vertically and one horizontally. However, such a method is disadvantageous with respect to the space factor in antenna systems for mobile stations.
On account of these circumstances, attempts have begun to be made in the past to use inverted-L antennas such as that shown in FIG. 1, or inverted-F antennas such as that shown in FIG. 2, instead of these monopole antennas. These antennas are easy to miniaturize, are of the type sensitive to magnetic fields and have an effect essentially similar to the polarization diversity effect.
FIG. 1(A) and FIG. 2(A) show the basic configurations of these inverted-L and inverted-F antennas of the past, and FIG. 1(B) and FIG. 2(B) show examples of actual antennas fabricated according to the basic configurations in each case.
Let us first explain the inverted-L antenna 10 shown in FIGS. 1(A) and (B). It consists of a vertical planar part 11 having a width W, and a horizontal planar part 12 which is bent at a right angle while being electrically connected at one end to this vertical planar part 11. The antenna is designed so that the sum of the length L of the horizontal planar part 12 and the height (or length) H of the vertical planar part 11 is equal to .lambda./4 with respect to the wavelength .lambda. of the frequency used. The feeding point P is located between the bottom of the vertical planar part 11 and the ground or earth E.
In the actual example of an antenna shown in FIG. 1(B), the ground E is configured on the upper surface of the shield housing (ground E) which shields the circuit parts (not shown in the drawing) which are assembled on a printed circuit board B. The inverted-L antenna 10 itself is also supported physically on this printed circuit board B. Of course, the vertical planar part 11, the horizontal planar part 12 and the shield housing E are made of conductive materials, generally suitable metals such as tinned steel sheets, and the printed circuit board B supporting them is made of an insulating materials such as glass epoxy.
The inverted-F antenna 20 shown in FIGS. 2(A) and (B), like the aforesaid inverted-L antenna 10, has a conductive horizontal planar part 22 with a length L and a conductive vertical planar part 21 with a height (or length) H positioned more or less at right angles towards each other, while the two parts are electrically connected to each other on one end. This antenna is also designed so that the sum of the aforesaid lengths (L+H) is equal to .lambda./4. However, the bottom of the vertical planar part 21 is directly connected to the ground E, which comprises the shield housing, and the feeding point P is led out from a position separated by a distance D from the connecting point of the vertical planar part 21 and the horizontal planar part 22, as is shown in FIG. 2(A).
As is shown in FIG. 2(B), the distance D can be considered by separating it into two parts: distances d.sub.1 and d.sub.2. In the inverted-F antenna 20 shown in the drawing, the vertical planar part 21 has a width q less than the width W of the horizontal planar part. This is for the purpose of improving the directivity. The usual practice is to design inverted-L antennas 10 or inverted-F antennas 20 so that the height H of the vertical planar parts 11, 21 is equal to about .lambda./10.
The inverted-L and inverted-F antennas shown in FIGS. 1 and 2 are superior in many respects to monopole antennas.
First of all, one may mention that their three-dimensional size can be made much smaller than that of monopole antennas. Moreover, they can coexist with the circuit parts mounted on a printed circuit board, as is shown in FIG. 1(B) and FIG. 2(B). Consequently, they can easily be housed inside the frame of communication equipment and can be miniaturized.
Second, although these inverted-L and inverted-F antennas 10, 20 are originally for use with vertically polarized waves, they also have horizontally polarized components, even though their radiation power has been reduced by about 20-30 dB. Therefore, even though they are single antennas, they have potentially a polarization diversity function.
However, a problem which tends to occur easily in the so-called plate antennas of this type of the past is the fact that it is difficult to match the impedance with the characteristic impedance of the feeder line.
For example, as mentioned above, the sum (L+H) of the height H of the vertical planar parts 11, 21 and the length L of the horizontal planar parts 12, 22 will necessarily be determined once the frequency f in use is determined. However, in most cases, it is desirable to reduce the height H of the vertical planar parts 11, 21.
In these cases, the antenna impedance generally tends to rise as the height H is reduced because of the increase of the parallel inductance. For this reason, mismatching of the impedance with the feeder line tends to occur easily.
Nevertheless, there are still ways of matching the impedance in these conventional antennas 10, 20 even if the height H is reduced. First, there is the method of adjusting the width W of the horizontal planar parts 12, 22. However, although there is no problem when this width W must be reduced, when it must be increased it becomes impossible to set it at the necessary width on account of the restrictions on the dimensions required in communication equipment. That is, there is not a very large degree of freedom in adjusting the impedance by adjusting the width W of the horizontal planar parts 12, 22.
On account of this, even among the conventional examples, if we compare the inverted-L antenna 10 shown in FIG. 1 with the inverted-F antenna shown in FIG. 2, one may say that the inverted-F antenna 20 shown in FIG. 2 is somewhat more advantageous with respect to adjustment of the impedance.
This is true for the following reason. In the inverted-L antenna 10 shown in FIG. 1, when the height H is restricted, one must rely solely on adjustment of the width W of the horizontal planar part 12 for adjusting the impedance. On the other hand, in the inverted-F antenna 20 shown in FIG. 2, even though both height H and width W may be restricted on account of dimensional requirements connected with miniaturization of the equipment, there still remains the means of adjusting the impedance by changing the lead-out position of the feeding point P, that is changing the distance D, or more realistically, by changing distances d.sub.1 and d.sub.2 in FIG. 2(B).
However, in actual fact, the range within which the impedance could be adjusted by these means was by no means sufficient. For this reason, restrictions were imposed on the dimensions of the equipment, and in most cases it was not possible to reduce the height H of the vertical planar part 21 very much.
In the case of the inverted-F antenna 20 in FIG. 2, which would seem to be somewhat superior to the inverted-L type, as mentioned above, there is an additional drawback in manufacturing of the equipment. That is, it becomes difficult to lead out the feeding point P when the distances d.sub.1, d.sub.2 concerning the feeding point P are adjusted in certain ways.